You should always use an escrow service before transacting with other people that you only know on the internet to prevent being scammed and the next time you buy Bitcoin use trustworthy exchanges not from random people. Anyway, yes you can reverse your payment on paypal if the status is unclaimed or pending, here's how you do it https://www.paypal.com/np/selfhelp/article/can-i-cancel-a-payment-that-i%27ve-already-sent-faq637 but if the receiver already claimed the money you cannot do anything about it and move on. It's your fault allowing this kind of transaction/trade.